In modern disk drive assemblies, mechanical clearance between the surface of the magnetic disk which stores data and the read/write head which transfers data to and from the disk is extremely small, i.e., about 10 microinches. Any particulate contamination on the disk surface or in the region of the read/write head is likely to cause a malfunction of the assembly. Consequently, the computer disk and the head must be enclosed and must be protected against contamination from the outside environment. The motor which drives the disk shaft and the bearings which support the disk shaft are sources of contamination due to their mechanically movable parts. However, it is generally inconvenient to enclose the motor, and contamination can arise from other sources and be carried into the disk drive region. Thus, a magnetic fluid exclusion seal is typically employed on the drive shaft to prevent contaminants from the motor and the bearings and from the external environment from reaching the interior of the disk assembly along the drive shaft.
The trend toward miniaturization of disk drives has required thinner and thinner drives, thereby severely limiting the axial space available for magnetic fluid seals and bearings and requiring designs with a very small axial length, or thickness. Magnetic fluid seals for use in small disk assemblies such as 51/4 inch and 31/2 inch spindles are typically required to have an axial thickness on the order of 1 millimeter.
Magnetic fluid, or ferrofluid, seals are generally known in the prior art. One prior art seal which is suitable for applications requiring a short axial length is a single pole piece ferrofluid seal disclose in U.S. Pat. No. 4,407,508, issued Oct. 4, 1983 to Raj et al. An annular pole piece surrounds the shaft to be sealed, and an annular permanent magnet of larger inside diameter than the pole piece is secured to one surface of the pole piece. The magnetic circuit extends through the magnet, the pole piece, a magnetic fluid in the gap between the pole piece and the shaft, through the shaft and is completed through the air gap between the shaft and the magnet. While such magnetic fluid sells provide satisfactory performance and a short axial length, the quantity of ferrofluid which is retained in the gap between the pole piece and the shaft is relatively small. As a result, the operational life of the seal is limited. Any attempt to increase the fluid quantity in the seal results in a ferrofluid splash from the seal region during operation.
Magnetic fluid seals including an annular magnet and two pole pieces, one on each end of the magnet, are known in the prior art. Two-pole seals having configurations with extended seal life are disclosed in U.S. Pat. Nos. 4,357,021, issued Nov. 2, 1982 to Raj et al and 4,357,022 issued Nov. 2, 1982 to Raj. These patents disclose seals utilizing pole pieces of unequal width to extend seal life. Although the two-pole piece configurations exhibit an improved seal life, the life may still be inadequate for certain applications such as disk drives operating at high ambient temperatures since the quantity of ferrofluid retained in the gaps between the pole pieces and the shaft is relatively small.
It is a general object of the present invention to provide improved magnetic fluid seals.
It is another object of the present invention to provide a compact magnetic fluid seal having long life.
It is a further object of the present invention to provide a compact magnetic fluid seal having a magnetic flux distribution which limits magnetic fluid splashing.
It is still another object of the present invention to provide a compact magnetic fluid seal which retains a relatively large volume of magnetic fluid.
It is still another object of the present invention to increase the life of a compact magnetic fluid seal without substantially reducing the pressure capacity or increasing the cost thereof.